In color photographic elements, a dye image is formed as a consequence of silver halide development by one of several different processes. The most common is to allow a by-product of silver halide development, oxidized silver halide developing agent, to react with a dye-forming compound called a coupler. The silver and unreacted silver halide are then removed to form the photographic element, leaving a dye image.
Such formation of a dye image commonly involves liquid processing with aqueous solutions that must penetrate the surface of the element to come into contact with silver halide and coupler. Thus, gelatin, and similar natural or synthetic hydrophilic polymers, have proven to be the binders of choice for silver halide photographic elements. Unfortunately, when gelatin, and similar polymers, are formulated so as to facilitate contact between the silver halide crystal and aqueous processing solutions, they are not as tough and mar-resistant as would be desired for something that is handled in the way that an imaged photographic element may be handled. Thus, the imaged element can be easily marked by finger elements, it can be scratched or torn and it can swell or otherwise deform when it is contacted with liquids.
Various techniques have been suggested to protect photographic elements from physical damage. One is to apply to the surface of the developed photographic element a preformed layer of a polymer more physically robust than gelatin, for example by lamination. Another is to apply to the surface of the developed element a liquid composition which is cured to leave a tough polymer layer. Unfortunately, these techniques for protecting the surface of a photographic element suffer from one or more problems.
Lamination has several disadvantages. For example, lamination involves an added expense associated with coating an additional support. Also, it is susceptible to trapping pockets of air between the laminate and the element during the laminating step leading to image defects. Moreover, because the laminate is self-supporting before lamination, it is thicker than necessary, which is wasteful of materials and can cause the element to curl if it is applied to only one side of the element. Application of a liquid overcoat can avoid some of the problem associated with lamination, such as formation of air pockets. But it introduces other problems. For example, handling the liquid compositions can be messy and such compositions often contain environmentally undesirable solvents. Moreover, liquid coatings can be difficult to dry or can require a separate UV curing step.
Electrophotography entails forming an electrostatic charge pattern on a surface and then forming a pattern of a marking composition, called a marking particles, on that surface as a function of the location of the charge pattern. The resulting pattern of marking particles is made permanent on an image bearing surface by application of heat and/or pressure to cause the marking particles to fuse and adhere to the image bearing surface. It has been suggested from time to time to overcoat such marking particle patterns in various ways, such as with a clear marking particles to form a protective overcoat on a marking particles image. In U.S. Pat. No. 5,804,341 (issued Sep. 8, 1998, in the names of Bohan et al), it has been suggested to use electrostatic or electrophotographic technology to apply a protective overcoat to images derived from silver halide photographic elements.
This would provide a protective overcoat to imaged photographic elements by a simple dry technique that gives easily applied, relatively thin overcoat layers. It has been found that electrophotographic marking particle compositions can adhere to the hydrophilic surface of a photographic element and protect the surface of the image during normal handling. That is, an imaged photographic element, comprising a silver halide derived image in a hydrophilic binder, has in the presence of an electric field, charged, clear polymeric particles are applied to a major surface of the element, so as to cause the particles to adhere to the surface of the element. The clear polymeric particles are fused so as to cause them to form a continuous polymeric layer on the surface of the element. The thin protective overcoat for the picture elements is a clear electrophotographic marking particle polymer. However, the protected photographic elements have been limited, in surface finish to one gloss condition, usually a high gloss finish.